Answer : (c) 30º
Given OA = 3 cm ⇒ OC = 3 cm (radii of the circle)
Also AC = 3 cm ⇒ OA = OC = AC ⇒ ∆ AOC is equilateral
⇒ ∠AOC = 60°.
⇒ ∠ABC = \(\frac{1}{2}\) \(\times\) ∠AOC
= 30° (Angle subtended by an arc of a circle at the center is double the angle subtended by it at any point on the remaining part of the circle.)